The present invention relates to an exhaust gas recirculation control device of an engine.
Conventionally, it has been known to provide a high-pressure EGR passage and a low-pressure EGR passage to recirculate a part of exhaust gas from an engine, from an exhaust passage to an intake passage. For example, JP2007-291973A discloses such a configuration. The high-pressure EGR passage connects a part of the exhaust passage upstream of a turbine of an exhaust turbocharger disposed therein, to a part of the intake passage downstream of a compressor of the exhaust turbocharger disposed therein (more specifically, downstream of an intercooler disposed downstream of the compressor). Moreover, the low-pressure EGR passage connects a part of the exhaust passage downstream of the turbine to a part of the intake passage upstream of the compressor. A low-pressure EGR valve for changing a cross-sectional area of the low-pressure EGR passage is disposed in the low-pressure EGR passage, and a high-pressure EGR valve for changing a cross-sectional area of the high-pressure EGR passage is disposed in the high-pressure EGR passage.
In JP2007-291973A, when the engine is in an idle operation, a flow rate of low-pressure EGR gas in the low-pressure EGR passage is increased to remove a deposit caused by condensed water, so that a deformation of the compressor and clogging of the intercooler due to the deposit are prevented.
Meanwhile, when the engine is in the idle operation, by keeping the turbine of the exhaust turbocharger rotated, a responsiveness to start (accelerating performance) of the vehicle improves. To keep the turbine rotated, an amount of exhaust gas recirculated by the low-pressure EGR passage is preferable to be increased as much as possible.
However, when the amount of exhaust gas recirculated by the low-pressure EGR passage is increased, condensed water is easily produced as disclosed in JP2007-291973A, then when the idle operation continues for a long period of time, the condensed water accumulates in the intercooler, and then when the engine stops in this state, there is a possibility that the condensed water will freeze.